Programmable domestic water heating system

ABSTRACT

The invention relates to a method for heating water in a domestic water heating system, that comprises the steps of: (a) providing a water tank containing mw liters of water, (b) providing at least one temperature sensing unit in said water tank for sensing the temperature of the water in the tank: (c) providing a heating element in said water tank; (d) providing a control unit for activating said heating element, said control unit continuously receiving indication for the water temperature from said temperature sensing unit; (e) providing to said control unit a desired water temperature, and designating a time for using the water at said desired temperature; (f) knowing the current water temperature, the desired water temperature at said designated time, the power of the heating element, and the specific heat of the water, calculating by the control unit the heating period Δt needed for heating the water in the tank from the current temperature as measured by said temperature sensing unit, to the desired temperature; (g) periodically repeating said calculation and updating said calculated period Δt according to changes in the sensed water temperature; and (h) when the designated usage time is approaching, activating the heating element a Δt period before the said designated period.

FIELD OF THE INVENTION

[0001] The field of the invention generally relates to electrical homeappliances. More particularly, the invention relates to an improveddomestic water heating system.

BACKGROUND OF THE INVENTION

[0002] Hot water is an essential commodity in the modern world, and awater heating system is an appliance commonly used in householdsthroughout the world.

[0003] In some countries where the price of the energy is negligible, itis common to activate the water heating system all the day, resulting ina significant waste of energy.

[0004] In other countries, where energy is relatively expensive, solarenergy is used for heating the water. However, the solar energy cannotgenerally provide hot water 24 hours a day, 365 days a year, andtherefore complementary heating involving energy consumption isrequired. The source of energy for this purpose is, in most cases,electricity or gas.

[0005] In order to save energy, activation of the water heating is onlyas needed. However, in most water heating systems of the prior art, theuser is not provided with any indication regarding temperature of thewater in the tank, and moreover, he has no indication whatsoeverregarding how long the heating system has to be ON in order to providewater in the desired amount and temperature. Generally, this causes theuser to activate the heating element of the system a longer time thannecessary resulting in a waste of energy, or a shorter time thannecessary, resulting in a colder and insufficient amount of water thandesired. Furthermore, even after the water heating is presumablycompleted, the user has no indication of the water temperature in thetank, and must open the tap and wait a relatively long time forregulating the temperature, resulting in a waste of water.

[0006] The lack of accurate temperature indication of the water in thetank, and the inability to plan in advance the necessary water amountand temperature causes inconvenience, waste of energy and water. Incases wherein the user constantly activates the water heating throughoutthe day and night, there is even more energy waste, particularly intimes when there is no need for hot water. This energy waste is added tothe energy loss resulting from the temperature difference betweenenvironment and the water in the tank, which in many cases issignificant.

[0007] Of course there are times where a user requires a relativelyhotter temperature than in other times. In the systems of the prior art,the pre-planning of the water temperature is either unavailable, orunsatisfactory. In conventional water heating systems of the prior art,and particularly for safety purposes, there is an adjustable thermostatmounted in a pocket in the water tank, which senses the watertemperature, and disconnects the electrical supply when a pre-assignedmaximum temperature is reached. However, in this case, the regular userdoes not have access to the thermostat, or control over the pre-assignedtemperature.

[0008] Some other prior art systems comprise a timer, either electricalor mechanical, for setting the duration of the water heating.

[0009]FIG. 1 shows a hot water tank 1 commonly used in systems of theprior art. The water tank 1 comprises an electric heating unit 3 forsupplying energy to the water. Heating unit 3 is essentially a resistor,heated by an electric current flowing through it, and transferring heatto the surrounding water. The water tank further comprises in its lowerpart an inlet water pipe 8, and in its upper part an outlet water pipe9. Two optional water pipes 104 and 105 are included in those standardwater tanks that are designed to operate with solar heat collectors.Through pipe 105 cold water leave the tank to a solar collector, andthrough pipe 104 hot water enter the tank from the solar collector (notshown). Metal flange 2 at the bottom of the tank supports the heatingunit 3. Also supported by the flange is a metal sleeve 4, serving as apocket for a standard thermostat. Insulating layer 5 blocks heattransfer to the surroundings. Thin metal 10 encloses the tank and theinsulating layer 5. Remote ON/OFF switch 6, is usually located in aneasily accessed place, and generally comprises a red indication thatlights up when the switch is ON. When the switch is ON and the watertemperature rises to the preset temperature of the thermostat, thethermostat disconnects current to unit 3. When the water temperaturefalls below said preset temperature, the thermostat reconnects thecurrent to the heating element.

[0010]FIG. 1 also shows a prior art system that further comprises a heatconcentrator 7 in the water tank. The heat concentrator 7, which is usedonly in a vertically oriented tank, is a cup-like device made of anysuitable material, mechanically connected to the bottom of the watertank. The heat concentrator 7 has openings 19 at its lower part forenabling water passage into it, and at its upper part an additionaloutlet opening 20. The heat concentrator 7 encloses the heating unit 3anid the thermostat pocket 4. When the heating unit 3 is activated, hotwater in concentrator 7 flows to the upper opening 20, and cold waterflows through the lower openings 19 to the concentrator, creating watercirculation. Layers of hot water are therefore concentrated at the upperpart of the water tank. After a long period of heating, all the water inthe tank becomes sufficiently hot, and the water temperature indifferent parts of the tank is relatively homogeneous.

[0011] Generally, it is common to use a heat concentrator 7 in watertanks of 80 liters or more.

[0012] Prior art

[0013] U.S. Pat. No. 6,002,114, filed Sep. 15, 1998, discloses a waterheating system which comprises:

[0014] 1. A water tank with four heating elements;

[0015] 2. Temperature sensors for checking the temperature at the inletand outlet of the water tank;

[0016] 3. A sensor for checking the water flow rate at the inlet pipe ofthe tank;

[0017] 4. CPU receiving sensor indications, for activating/deactivatingsaid four heating elements, further comprising a circuitry for detectingfailures; and

[0018] 5. A display panel for showing the user the temperature of thewater leaving the tank.

[0019] More particularly, U.S. Pat. No. 6,002,114 deals with acommercial heating system having four electric heating elements, and aplurality of sensors. The heating elements are activated according towater temperature at the inlet and outlet of the tank, while furtherconsidering the inlet water flow rate.

[0020] DE 29719 267 discloses a microprocessor-based controller for anelectric water heating system. The front panel of the housing of thecontroller has several push buttons for setting the desired temperatureand various other parameters, for selecting from a function menu, andfor activating a rapid heating mode. The controller further comprises aseven-segment display with a temperature bar indicating the thermalstate of the heating system.

[0021] U.S. Pat. No. 5,556,564 discloses a domestic water heating systemhaving a unit for controlling the water temperature. The said systemcomprises:

[0022] 1. Three temperature sensors, a first sensor at the top, next tothe outlet of the water from the tank a second in the middle of thetank, and a third at the bottom of the tank next to the water inlet;

[0023] 2. A display panel showing the temperature measured by the uppersensor, and enabling the user to set the required temperature of waterleaving the tank;

[0024] 3. Two light indicators which deactivate when the middle, and thelower sensors measure temperatures above the set temperature. The lightindicators indicate to the user when there is enough water in the tankfor use.

[0025] 4. The hot water tank and the control panel are distant one fromthe other, and are connected by only two electric wires. The same twoelectric wires provide the power to the heating element, and transferthe low voltage temperature indication from the upper sensor in the tankto the control panel.

[0026] FR 2 539 238 discloses a control method and device for anapparatus for heating a fluid to reach a predetermined temperature. Thedevice comprises a central control unit receiving a signal from atemperature probe which identifies the temperature of the fluid, astorage unit for storing a characteristics data of the apparatus used,and a circuit for setting a predetermined temperature. The invention isparticularly useful in electric water heating systems. The system ofthis patent particularly intends to activate the heating doing low-rateelectrical periods, for example, overnight, weekends, etc. This patentidentifies the periods of low-cost electric energy in order to activatethe heating particularly during these periods. The system follows theexpressions: th=(TF′−TD); and 1<K+ta. th is the temperature at the endof the low cost electric energy period. TF′ is the time at the end ofthe low cost electric energy, TD is the present time, K is a factordescribing the intensity of the electric power at the heating elementand the water volume in the tank. This formula cannot determine the timerequired for heating the water in the tank. All this is available forone cycle a day. The system also enables manual heat activation fortimes when the energy cost is higher.

[0027] U.S. Pat. No. 4,568,821 discloses still another remote waterheating system. The system comprises two water tanks, one tank solarheated, the other heated by electricity, oil or gas. The systemcomprises two temperature sensors located at the outlet pipes of eachwater tank. The controller of said system uses a 24-hour clock, and isassembled with solid state electronic components.

[0028] All the above prior art systems are designed to provide bettercontrol over water heating systems, and to save energy. Some of theprior art systems allow the designating of a period for heating with astarting time. However, these systems do not consider the watertemperature at the starting time for heating, in which the water isheated for the said designated period, resulting in hotter water thannecessary (and waste of energy) or colder than necessary (resulting ininconvenience). In some other cases, the water reaches the desiredtemperature before the time planned for use, and the heating terminates.However, until the water is actually used, the temperature decreases,resulting in a waste of energy and inconvenience. The water heatingsystem of the invention provides more energy and water saving incomparison with the prior art water heating systems, a manner forefficient installation, and also more convenience for the hot wateruser.

[0029] The present invention also discloses a new, efficient and easymanner of assembling temperature sensing units in a water tank, asrequired by the system of the invention, therefore obtaining moreaccurate temperature sensing, and improving even more the energy andwater saving. Such a manner of assembling the system of the invention isapplicable in both existing water heating systems or in newly installedwater heating systems

[0030] It is therefore an object of the invention to increase energy andwater savings in a domestic water heating system.

[0031] It is another object of the invention to provide to the userbetter control and more reliable indications relating to the temperatureof the water in the tank.

[0032] It is still another object of the invention to enable easyinstallation of the system of the invention, in existing water heatingsystems, on site.

[0033] It is still another object of the invention to provide electricaland electronic failure indications, by visual or audible means.

[0034] It is still another object of the invention to provide an easymanner of installation of the system of the invention in new or existingwater heating systems. This manner of assembling relates particularly tothe introduction and assembling of temperature sensing units in thetank.

[0035] It is still another object of the invention to provide new mannerby which data is communicated between the temperature sensing unit's ofthe tank generally located outside of the house, and the control unitlocated inside.

SUMMARY OF THE INVENTION

[0036] The present invention relates to a method for heating water in adomestic water heating system which comprises: (a) Providing a watertank containing m_(w) liters of water; (b) Providing at least onetemperature unit in said water tank for sensing the temperature of thewater in the tank; (c) Providing a heating element in said water tank;(d) Providing a control unit for activating said heating element, saidcontrol unit continuously receiving indication of the water temperaturefrom said temperature sensing unit; (e) Providing to sad control unit adesired water temperature, and designating a time for using the water atsaid desired temperature; (f) Knowing the current water temperature, thedesired water temperature at said designated time, the power of theheating element and the specific heat of the water, calculating by thecontrol unit the heating period Δt needed for heating the water in thetank from the current temperature as measured by said temperaturesensing unit to the desired temperature; g) Periodically repeating saidcalculation and updating said calculated period Δt according to changesin the sensed water temperature; (h) When the designated usage time isapproaching, activating the heating element a Δt period before the saiddesignated usage time.

[0037] Preferably, the calculation further includes a consideration ofthe a heat loss factor.

[0038] In an embodiment of the invention, the following formula is usedby the control unit:${\Delta \quad t} = \frac{m_{w}C_{p}\Delta \quad T}{P_{H}}$

[0039] Wherein:

[0040] Δt is the expected heating period by the heating element[seconds];

[0041] P_(H) is the power of the heating element [Watts];

[0042] m_(w) is the volume of the water in the water tank measured inliters;

[0043] C_(p) is the specific heat capacity of the water$\left( {= {4200\quad \frac{Joules}{{{kg} \cdot {\,{\,{\,{^\circ}}}}}\quad {C.}}}} \right);$

[0044] ΔT is the difference between the designated temperature at alater desired time and the current temperature of the water in the tank,measured in degrees Celsius [° C.).]

[0045] According to another embodiment of the invention the calculationis made by the following formula:${\Delta \quad t} = \frac{{Km}_{w}C_{p}\Delta \quad T}{P_{H}}$

[0046] wherein K is the loss factor;

[0047] Preferably, the loss factor K is calculated by the followingformula:$K = {1 + {\frac{m_{v}}{A} \cdot \frac{\Delta \quad T}{B} \cdot \frac{C}{P_{H}}}}$

[0048] Wherein:

[0049] A is the volume of the tank used [in liters], B is the differencebetween the required temperature in the tank and the air temperaturesurrounding the tank [in ° C. ], and C is the heat lost to thesurroundings, [in Watts], as acquired by experimental results.

[0050] The invention also relates to a water heating system, whichcomprises: (a) a water tank; (b) a heating element in said water tank;(c) at least one temperature sensing unit for sensing the temperature ofthe water in the tank; (d) a control unit located in a place accessibleto the user, the control unit receives from said temperature-sensingunit an indication to the current temperature, The control unit furthercomprises: I) a display for displaying the current water temperature asacquired by the said temperature sensing unit; (II) a display and pushbuttons allowing the user to designate time for having hot water at adesired water temperature; (III) calculating means for calculating fromthe current water temperature, the desired water temperature, the powerof the heating element and the specific heat of the water a heatingperiod in which the heating element has to be activated in order to heatthe water to the desired water temperature by the heating element; and(IV) switching means for providing voltage to the heating element duringsaid calculated heating period.

[0051] Preferably, the temperature-sensing unit comprises at least onetemperature sensor having means for transforming a change in temperatureinto a proportional change in voltage.

[0052] Preferably, the said system comprises at least one temperaturesensing unit in a form of a tube within the hollow of which at least onetemperature sensor is mounted.

[0053] Preferably, each temperature sensing unit is introduced into thespace of the water tank from within an opening in one of the pipesleading water to or from the tank, and wherein said opening is thensealed in such a manner as to prevent leakage of water through saidopening while letting the temperature sensing unit containingmeasurement wires coming from the sensor/s to penetrate through thesealing.

[0054] Preferably, a T-type connector is connected to the pipe with thetemperature sensing unit, one end of said T connector forms the saidopening with sealing, the other two ends of the said T connector leadwater to or from the tank.

[0055] Preferably, a cap with a bore is used at the said opening, thebore being sealed by a sealing material, while letting said temperaturesensing unit containing measurement wires coming from the sensor/s topenetrate through the sealing.

[0056] Preferably, the pipe through which the temperature-sensing unitis introduced into the tank is the pipe leading hot water out from thetank.

[0057] Preferably, one temperature sensor is located at the distal endof the temperature-sensing unit, away from the sealed opening and withinthe space of the tank. In another option, a plurality of temperaturesensors may be mounted along the unit, to measure temperatures atdifferent levels of the water in the tank.

[0058] Preferably, each temperature sensor provides transformation of achange in temperature into a proportional change in voltage.

[0059] In still another embodiment of the invention, twoline-transceivers are used, one at a location close to the tank, and theother at a location close to or within the casing of the control unit,for providing transfer of data relating to the temperature of the waterin the tank to the control unit, and data from the control unit to anactuator of the heating element located next to the heating element,over the electricity lines supplying current to the heating element. Inanother alternative, two transceivers are used, one at a location closeto the tank, and the other at a location close to or within the casingof the control unit, for providing wireless transfer of data relating tothe temperature of the water in the tank to the control unit, and datafrom the control unit to an actuator of the heating element located nextto the heating element.

BRIEF DESCRIPTION OF THE DRAWINGS

[0060]FIG. 1 is a schematic depiction of a domestic water heating systemaccording to the prior art,

[0061]FIG. 2 illustrates an installation of a temperature-sensing unitin a water tank of the type of FIG. 1;

[0062]FIG. 3 illustrates an installation of a temperature-sensing unitin a water tank of the type of FIG. 1;

[0063]FIG. 4A illustrates a temperature sensing unit with onetemperature sensor inside, according to a first embodiment of theinvention;

[0064]FIG. 4B illustrates a temperature sensing unit with threetemperature sensors inside, according to a second embodiment of theinvention.

[0065]FIG. 5 describes an exemplary front panel of the control unit,including the display and push buttons according to the first (mostcommon) alternative

[0066]FIG. 6 shows an embodiment of the invention in which the controlunit is split into 2 parts, using transceivers for conveying informationbetween the two parts;

[0067]FIG. 7A shows an embodiment of the invention in which a computercommands the control unit; and

[0068]FIG. 7B shows another embodiment of the invention in which acomputer controls an embodiment as shown in FIG. 6;

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0069] The invention provides improvements to domestic water heatingsystems. More particularly, the system of the invention provides animproved control over the water heating, enabling the user to plan anddefine in advance the exact temperature of the water in the water tank,and the time at which heated water will be needed at the definedtemperature. As said, some of the domestic heating systems of the priorart enable the defining of a desired water temperature at a specifictime. However, these systems are either not sufficiently accurate,particularly in determining the exact temperature of the mass amount ofthe water, or are not optimized in their energy consumption.

[0070] The following equations are used in the control unit of thesystem for defining the required heating period, and the exact startingtime in which the heating is initiated:

Watts·sec=m _(w) ·C _(p) ·ΔT   (1)

[0071] Wherein:

[0072] Watt·sec is the heating energy provided to the water by theheating element;

[0073] m_(w) is the volume of the water in the water tank measured inliters (=Kg);

[0074] C_(p) is the specific heat capacity of the water$\left( {= {4200\quad \frac{Jouls}{{{kg} \cdot {^\circ}}\quad {C.}}}} \right);$

[0075] ΔT is the difference between the desired temperature at a laterdesired time and the present temperature of the water in the tank,measured in degrees Celsius [° C.]. $\begin{matrix}{{\Delta \quad t} = \frac{K \cdot {Watt} \cdot \sec}{P_{H}}} & (2)\end{matrix}$

[0076] wherein:

[0077] Δt is the expected heating duration of the heating element[seconds];

[0078] P_(H) is the power of the heating element installed in the watertank [Watts];

[0079] K is an experimental factor which enables the algorithm toaccurately calculate the time duration required to heat the water in thetank to the desired temperature. Formula (3) details how K iscalculated. It is partially based on actual experiments that wereperformed by the inventors, taking into account the volume of the watertank (m_(w)), the temperature difference (ΔT), and the power of theheating element (P_(H)).

[0080] The control unit calculates the value of K according to thefollowing formula: $\begin{matrix}{K = {1 + {\frac{m_{W}}{A} \cdot \frac{\Delta \quad T}{B} \cdot \frac{C}{P_{H}}}}} & (3)\end{matrix}$

[0081] A, B, and C are numerical values obtained by laboratoryexperiments. A=60 liters, is the volume of the tank used, B=20° C. isthe difference between the required temperature in the tank and the airtemperature surrounding the thermally insulated tank. C=70 Watts was theheat lost to the surroundings. These values may change by accumulationof experience, and with variations in materials and structure of thewater tank.

EXAMPLE 1

[0082] An 80 liter water tank having a heating element of 2500 Watts isprovided. The present temperature of the water in the tank is 28° C. Itis desired that at 19:00 this evening, the water temperature will be 50°C.

ΔT=50−28=22° C.

[0083] Therefore:

Watts·sec=80·4200·(500−28)=7.392·10⁶ Joules

[0084] $\begin{matrix}{K = {{1 + {\frac{80}{60} \cdot \frac{22}{20} \cdot \frac{70}{2500}}} = 1.041}} \\{{\Delta \quad t} = {\frac{1.041 \cdot 7.392 \cdot 10^{6}}{2500} = {{3078.2\quad \sec} \cong {52\quad \min}}}}\end{matrix}$

[0085] Therefore, the heating element will be activated at 18:08:00. Ifthe user desires, the program may be set to continue water-heating for aspecified duration of time. For example, if the user desires to keep thewater in the tank at this temperature for an additional 40 minutes, theheating will resume each time the water temperature drops below 50° C.,until 19:40.

[0086] According to the invention, the control unit operatescontinuously, checks the present date, time and temperature of the waterin the tank, and calculates when to activate the heating element.

[0087] Example 1 shows that the system saves a significant amount ofenergy in comparison to systems of the prior art which includemechanical/electrical/electronic timers that do not consider the presenttemperature before activating the heating process. The systems of theprior art thus maintain water in the tank at higher temperatures thanneeded over long periods. The advantage of the algorithm of theinvention is that the water in the tank is heated only towards therequired time, in order to reach the exact desired temperature preciselyat the set time, and thus heat loss to the environment is minimized.Therefore, the system of the invention provides appreciable energysavings.

[0088] The temperature of the water in the tank is measured by a sensingunit mounted in the tank, with data continuously provided to the controlunit. The user defines the times, desired temperature, and the timeduration to keep the said temperature. The control unit is located in aplace convenient to the user and remote from the hot water tank.

[0089] In still another embodiment of the invention, the user may defineinstead of the temperature another temperature related, or water volumerelated indication, such as the number of showers he plans to use.

[0090] The measured water temperature or another temperature relatedindication is displayed continuously on a front panel of the controlunit. The user introduces to the control unit by means of push buttonsthe desired settings. For example, the user may set a required watertemperature, a date and time in which the required water temperature isdesired, and the duration for which this temperature is desired.

[0091] The control unit retains the user settings in an internal memory.The user can also activate or deactivate the heating directly, or thetimer operation of the control unit.

[0092] The preferred water temperature sensing unit according to theinvention is, for example, PTX type sensor (PT stands for PlatinumTemperature, X defines the type of thermistor, like 100, 1000), athermocouple sensor, a digital thermometer, or any other equivalenttemperature-sensing element. Installation of all parts of the system(such as the control unit, the temperature sensing unit, and wires) issimple, and any existing standard domestic water heating system can beupgraded to the system of the invention with relative ease.

[0093] The control unit preferably also includes the option of failuredetection, which alerts the user of detected failures, such as in theheating element or in the temperature-sensing unit, or the safetydevices. Any of the above failures causes automatic termination of thevoltage supply to the electric heating element.

[0094] In order to provide best performance of the system of theinvention, it is essential to obtain an accurate indication of the watertemperature in the tank. In a preferable embodiment of the system of thepresent invention, a single temperature sensing unit is installed in thewater tank. FIGS. 2 and 3 show in schematic form the system according toone embodiment of the invention. A temperature sensing unit 17A or 17Bis introduced into the water tank through a water pipe 9 or 104, whichis an integral part of the water tank 1.

[0095] It should be noted that the temperature-sensing unit 17A or 17Bmay also be introduced into the water tank in any conventional manner.

[0096]FIGS. 2 and 3 show two alternatives by which thetemperature-sensing unit is introduced into tank 1 through water pipe 9or water pipe 104. Sealing nuts 102A in FIG. 2 and 102 1B in FIG. 3 areused for enabling penetration of the temperature; sensing unit throughit, while sealing water leakage.

[0097] As said, the temperature sensing units 17A and 17B in FIGS. 2, 3and 4 preferably include a PTX-type sensor, a thermocouple, a digitalthermometer, or an equivalent device. This is an important part of theinvention, as the immersion of the temperature sensing unit in the waterin the tank results in an accurate measurement, and the method ofpenetration enables easy installation of the temperature sensing unit instandard water tanks. FIG. 4A and FIG. 4B detail two devices: FIG. 4Ashows a temperature sensing unit 17A with only one sensor 107 and FIG.4B shows a sensing unit 17B in a form of a one metal sleeve withmultiple temperature sensors 107A, 107B, and 107C installed inside. Thesensing unit penetrates through the nuts 102A in FIG. 2 and 102B in FIG.3 and the nut seals the penetration area. The connecting wires 109 inFIG. 4A, or 109A, 109B and 109C in FIG. 4B direct the temperaturemeasurements of the sensor/s to the control unit.

[0098] Some Observations on Temperature Sensor/Sensors:

[0099] a. According to an embodiment of the present invention, more thanone temperature-sensing unit 17A or 17B can be installed in the watertank.

[0100] b. The invention also discloses a domestic water heating systemwith an improved manner and accuracy of sensing the temperature of thewater in the water tank. FIGS. 2 and 3 illustrate two alternatives ofinstalling one temperature sensing unit in the water-heating tank.

[0101] c. One or more sensing units can be installed at differentlocations in the tank to directly measure the water temperature. If morethan one sensor is used within one or more sensing units, the algorithmis provided with information relating to which of the sensors or acombination thereof to use, and at what time

[0102] In FIG. 3 a temperature sensing unit 17B is inserted via anexisting water pipe 104 into the water tank 1. In some cases, forexample, when solar heat collectors are not in use, there may existunused pipes, in this case pipes 104 and 105, connected to the tank, andare sealed by a cap. According to the invention, a cap is removed, and atemperature-sensing unit 17B is inserted through pipe 104. Thetemperature-sensing unit may include one or more temperature sensors,for measuring the temperature of the water, at different levels withinthe tank. Each temperature-sensor is connected to at least two wires forproviding electronic indications regarding the temperature it measures.The wires of the temperature sensors are connected to a control circuit(not shown) that controls the activation of the heating system. The cap102B is preferably a hexagon-shaped cap having a bore 75 in its center.A conventional sealing material is used for sealing around the tubeforming the temperature-sensing unit, preventing the passage of waterout of the tank.

[0103] It should be noted that the diameter or the temperature sensingunit 17B is generally much smaller in comparison with the diameter ofpipe 104, essentially in the range of no more than ⅓ or ¼ ofthe,diameter of the pipe.

[0104] The temperature-sensing unit 17A, including the one or moretemperature sensors, can optionally be introduced into the water tankvia a pipe in use. FIG. 2 shows such a case in which thetemperature-sensing unit 17A is introduced into the water tank via theoutlet of hot water pipe 9. In that case, a T-connector 101 is used forenabling the introduction, through a first side of it (the sideconnected to 102A), of the temperature sensing unit 17A into the watertank, while allowing the regular flow of water to pass into the hotwater supply pipe 103C. The cap 102A is preferably a hexagon-type cap,similar to the cap 102B of FIG. 3, with a bore 77 through which thetemperature sensing unit passes. A sealing material is used to preventleakage of water through bore 77. The diameter of the temperaturesensing unit 17A is essentially small in comparison with the diameter ofthe pipe 9, not to significantly disturb the flow of the water throughpipe 9. It has been found by the inventors that a unit diameter of up toabout ⅓ of pipe 9 diameter does not cause a significant disturbance tothe flow of water through pipe 9.

[0105] It should be noted that the term temperature sensing unit usedherein refers to any type of temperature measuring means.

[0106]FIGS. 4A and 4B illustrate how the temperature sensors 107 areassembled within temperature sensing unit 17. In FIG. 4A, onetemperature sensor 107 is assembled within a temperature-sensing unit17A. The sensor itself is indicated as numeral 107, and numeral 108indicates a sleeve that encloses the electrical wires 109, that areconnected to a control circuit that controls the activation of theheating when necessary. The upper portion of temperature sensing unit17A is positioned in the water tank, and the wires 109 are outside thewater tank, being connected to the control circuit (not shown). FIG. 4Bsimilarly illustrates how multiple sensors, for example three sensors107A, 107B, and 107C are assembled within the temperature-sensing unit17B, at different heights, for enabling the temperature measurement atdifferent levels of the water within the tank.

[0107] Preferably, the temperature sensors are of the type PTX, or adigital thermometer, having each between two to four output wires 109.

[0108] According to a preferred embodiment of the invention the existingthermostat located in pocket 4 of the tank 1 is used only as a safetydevice to terminate the electric current flow in case the maximal valueset for the water temperature in the tank is exceeded.

[0109] As said, in a preferable embodiment of the invention theinsertion of the temperature sensing unit is made through an existingopening of a water pipe. Moreover, the temperature measuring has beenfound by the inventors to be much more accurate due to the followingreasons:

[0110] a. The temperature sensing unit is inserted inside the water tankand preferably, there is a direct contact between the sensing device andthe water.

[0111] b. One or more temperature sensors can be designed to be mountedessentially at any height, and at almost any location within the watertank.

[0112] The invention provides a method for introducing one or moretemperature sensors within one or more temperature sensing units into awater tank. The manner of such introduction is useful in both existingwater tanks and in future water tanks. In the first case, suchintroduction of the temperature sensing unit/s provides a more accuratemeasuring. In the latter case, such introduction of the temperaturesensing unit/s also eliminates the need to provide a dedicated pocketfor a temperature-sensing device, therefore reducing the cost ofproduction of the tank. Moreover, such manner of introduction is simple,and can be easily carried out into practice at low cost.

[0113] The present invention requires a transfer of temperature datafrom the temperature sensing unit/s that is frequently located remotely,for example, on the roof of the house (or building), while the controlunit is generally located inside the home of the user. Furthermore, itrequires the transfer of electricity from the control unit to theheating element at the tank. This generally requires the introduction ofat least two additional wires for conveying data from the water tank tothe control unit and vice versa. When installing the system of thepresent invention in new houses, this involves generally only slightlyadditional costs. However, when upgrading heating systems of the priorart to operate according to the invention, the introduction of the twoadditional wires is a relatively complicated task. The present inventionprovides a solution also to this problem. According to a preferredembodiment of the invention two transceivers are introduced, one in theroof, and one inside the house to convey data information between theroof and the control unit over the electrical lines leading electricityfrom the control unit to the heating element. Therefore, according tothis embodiment the same electrical lines are used both for the transferof electricity to the heating element, and both for the transfer oftemperature information from the tank to the control unit. Suchtransceivers are known in the art. For example, transceivers of the typeTDA 5051 by Philips Company can be used.

[0114] General: The control unit comprises a display, software foroperating the unit, electronic components, and electrical and mechanicalcomponents. The algorithm according to which the unit operates is basedon the formulas as given hereinbefore. As said, the algorithm uses atleast three main parameters in order to calculate when and for how longto activate the heating: (a) the water temperature before the heating;(b) the known specific heat of the water; (c) the desired watertemperature at the time when the hot water is to be used; and (d) theknown power of the heating element used.

[0115] Hereinafter, several variants and examples of the invention willbe described.

[0116] 1^(st) variant: a standard system, the control unit includingsoftware for enabling two modes of operation, manual or automatic.

[0117] Manual activation is provided by setting the ON/OFF switch 23 inFIG. 5 to the ON position, thereby enabling the control unit to activatethe heating element. The heating terminates either by manually turningOFF switch 23, or by the control unit when the desired water temperaturein the tank, as programmed by the user, has been reached The programmingof the control unit is performed by the user, using the display and thebuttons of the unit.

[0118] It should be noted that this is a basic alternative of thecontrol unit and it can be modified by means of software and/or hardwareto be even more user-friendly.

[0119]FIG. 5 shows a first possible structure for control panel 21 ofthe control unit. Control panel 21 comprises a numeric display 22,showing the time 22A (hour:minutes), and the current temperature in thetank 22B (or another display related to the water temperature e.g.number of showers). Switch 23 activates the heating system When switch23 is ON, the control unit operates, to activate the heating elementwhen needed. The digital display 22 functions always, whether switch 23is ON or OFF. Red light 24 is activated when current flows through theheating element and it turns OFF when no current passes through it. Thefirst push button 26 is used for setting the current time. The secondpush button 25 functions as follows: when activated, the threshold valueof the desired water temperature appears on display 22B. The third pushbutton 27 functions as follows: when activated, the desired time atwhich water at said threshold temperature is needed appears on thedisplay 22A The fourth push button 28 functions as follows: whenactivated, display 22A registers the additional duration at which thethreshold temperature should exist in the water tank. The fifth andsixth push buttons 29A and 29B are adjustment buttons, button 29A upwardand button 29B downward, respectively.

[0120] a. When pushing button 25 and one of buttons 29A or 29B thethreshold temperature changes on the display 22B: Note that thethreshold temperature cannot exceed a preset value, 65° C. for example,particularly for the sake of safety.

[0121] b. When pushing button 26 and one of buttons 29A or 29B, thecurrent time can be adjusted.

[0122] c. When pushing button 27 and one of buttons 29A or 29B, thedesired time for using the water can be adjusted.

[0123] d. When pushing button 28 and one of buttons 29A or 29B, theadditional duration for which the threshold temperature should bemaintained in the tank can be adjusted.

[0124] 2^(nd) Variant: In this alternative the control unit is dividedinto two parts, the first part is located in convenient location for theuser and it will be called part A 21A and the second will be called partB 21B in FIG. 6 and is located very near the water tank Thecommunication between part A and part B (in both directions) isperformed by means of a Current Transceiver, a C Bus or another standardelectronic device which is capable of communicating digital informationbetween two control units, which use the power lines of one phaseserving the heating element 3 of the water tank. Within part A are thewater temperature display (or another display related to watertemperature, e.g. number of showers), time display, the differentpush-buttons, software and a digital information transmitter/receiver.In part B is the final component which delivers current to the heatingelement 3 in the water tank, the water temperature electronic systemconnection to the temperature sensing unit 17 and a digital informationtransmitter/receiver.

[0125] 3^(rd) Variant: FIG. 7A shows a configuration that comprises aconventional computer (PC). The control unit 21 is connected to a PC 81via any conventional communication means. Any setup of the control unit21, can be performed from the PC, and the information relating to thecurrent status of the water tank can be transferred and displayed on thescreen of the PC. For that purpose, a dedicated software should resideat the PC.

[0126] 4^(th) Variant: FIG. 7B shows a variant of the invention, inwhich the whole control unit is embodied by a PC. The communicationbetween the PC and the tank is carried out by means of transceivers 21Aand 21B, that transfer data over the electricity lines.

EXAMPLE 2

[0127] It is desired to have water in the tank in a temperature of 50°C. at 19:00. The present temperature in the tank is 28° C. The controlunit uses the algorithm to calculate the time duration required for theheating element to heat the water up to 50° C. The present calculation,using the algorithm of formula (2), results in 52 minutes of heating.The software continuously checks the calculation until 19:00 minus 52minutes=18:08. At the calculated time 18:08 (if the temperature in thetank is still 28° C.), the heating element is activated automatically bythe control unit. At 19:00, when the temperature reaches the desiredtemperature of 50° C., the control unit terminates the heating. Ifduring the heating, i.e., between 18:08 and 19:00, hot water is consumedfrom the tank, and therefore the water temperature at 19:00 is found tobe lower than the desired, the control unit continues to activate theheating element, until the water temperature reaches the desiredtemperature. Furthermore, the user may be provided with the option ofprogramming the unit to continue providing hot water at 50° C. in aconsuming duration of, for example, 40 minutes. In that case, the waterwill be heated to 50° C. at 19:00, and any time between 19:00 and 19:40when the temperature drops below 50° C., the control unit activates theheating element 3.

[0128] While some embodiments have been illustrated by means of theabove examples, it should be understood that the invention may becarried out with many variations, modifications and adaptations, withoutdeparting from its spirit or exceeding the scope of the claims.

1. A method for heating water in a domestic water heating system,comprising: Providing a water tank containing mw liter, of water;Providing at least one temperature-sensing unit in said water tank forsensing the temperature of the water in the tank; Providing a heatingelement in said water tank; Providing a control unit for activating saidheating element, said control unit continuously receiving indication forthe water temperature from said temperature sensing unit; Providing tosaid control it a desired water temperature, and designating a time forusing the water at said desired temperature; Knowing the current watertemperature, the desired water temperature at said designated time, thepower of the heating element, and the specific heat of the water,calculating by the control unit the heating period Δt needed for heatingthe water in the tank from the current temperature as measured by saidtemperature sensing unit, to the desired temperature; Periodicallyrepeating said calculation and updating said calculated period Δtaccording to changes in the sensed water temperature; and When thedesignated usage time is approaching, activating the heating element aΔt period before the said designated usage time.
 2. A method accordingto claim 1, further including in the calculation a heat loss factor. 3.A method according to claim 1, wherein the formula used by the controlunit is:${\Delta \quad t} = \frac{m_{w}C_{p}\Delta \quad T}{P_{H}}$

Wherein: Δt is the expected heating period by the heating element[seconds]; P_(H) is the power of the heating element [Watts]; m_(w) isthe volume of the water in the water tank measured in liters; C_(p) isthe specific heat capacity of the water$\left( {= {4200\quad \frac{Joules}{{{kg} \cdot {\,{\,{\,{^\circ}}}}}\quad {C.}}}} \right);$

ΔT is the difference between the designated temperature at a laterdesired time and the current temperature of the water in the tank,measured in degrees Celsius [° C.].
 4. A method according to claim 2,wherein the calculation is made by the following formula:${\Delta \quad t} = \frac{{Km}_{w}C_{P}\Delta \quad T}{P_{H}}$

wherein K is the loss factor;
 5. A method according to claim 4, whereinthe loss factor K is calculated by the following formula:$K = {1 + {\frac{m_{w}}{A} \cdot \frac{\Delta \quad T}{B} \cdot \frac{C}{P_{H}}}}$

Wherein: A is the volume of the tank used [in liters], B is thedifference between the required temperature in the tank and the airtemperature surrounding the tank [in ° C. ], and C is the heat lost tothe surroundings [in Watts], as acquired by experimental results.
 6. Adomestic water heating system comprising: a water tank; a heatingelement in said water tank; at least one temperature sensing unit forsensing the temperature of the water in the tank; a control unit locatedin a place accessible to the user, the control unit receives from saidtemperature-sensing unit an indication to the current temperature, thecontrol unit further comprises: a. a display for displaying the currentwater temperature as acquired by the said temperature sensing unit; b. adisplay and push buttons allowing the user to designate time for havinghot water at a desired water temperature; c. calculating means forcalculating from the current water temperature, the desired watertemperature, the power of the heating element and the specific heat ofthe water a heating period in which the heating element has to beactivated in order to heat the water to the desired water temperature bythe heating element; and d. switching means for providing voltage to theheating element during said calculated heating period.
 7. A domesticwater heating system according to claim 6 wherein thetemperature-sensing unit comprises at least one temperature sensor,having means for transforming a change in temperature into aproportional change in voltage.
 8. A domestic water heating systemaccording to claim 6 comprising at least one temperature sensing unit ina form of a tube within the hollow of which at least one temperaturesensor is mounted.
 9. A heating system according to claim 8, whereineach temperature sensing unit is introduced into the space of the watertank from within an opening in one of the pipes leading water to or fromthe tank, and wherein said opening is then sealed in such a manner as toprevent leakage of water through said opening while letting thetemperature sensing unit containing measurement wires coming from thesensor/s to penetrate through the sealing.
 10. A system according toclaim 9 wherein a T connector is connected to the pipe with thetemperature sensing unit, one end of said T connector forms the saidopening with sealing, the other two ends of the said T connector leadwater to or from the tank
 11. A system according to claim 9 wherein acap with a bore is used at the said opening, the bore being sealed by asealing material, while letting said temperature sensing unit containingmeasurement wires coming from the sensor/s to penetrate through thesealing.
 12. A system according to claim 9 wherein the pipe throughwhich the temperature-sensing unit is introduced into the tank is thepipe leading hot water out from the tank.
 13. A system according toclaim 9 wherein one temperature sensor is located at the distal end ofthe temperature-sensing unit, away from the sealed opening and withinthe space of the tank.
 14. A system according to claim 9 wherein aplurality of temperature sensors are mounted along the unit, to measuretemperatures at different levels of the water in the tank.
 15. A systemaccording to claim 9, wherein each temperature sensor transforms achange in temperature into a proportional change in voltage.
 16. Asystem according to claim 6, further comprising two line-transceivers,one at a location close to the tank, and the other at a location closeto or within the casing of the control unit, for providing transfer ofdata relating to the temperature of the water in the tank to the controlunit, and data from the control unit to an actuator of the heatingelement located next to the heating element, over the electricity linessupplying current to the heating element.
 17. A system according toclaim 6, further comprising two transceivers, one at a location close tothe tank, and the other at a location close to or within the casing ofthe control unit, for providing wireless transfer of data relating tothe temperature of the water in the tank to the control unit, and datafrom the control unit to an actuator of the heating element located nextto the heating element.